Control of selective catalytic reduction (SCR) catalysts is of increasing interest to meet modern internal combustion engine emissions standards. The efficiency of a typical SCR catalyst in removing NOx emissions is sensitive to the molar ratio of NO2 to NOx at the inlet to the SCR catalyst. Under most conditions, the ratio of NO2 to NOx at the SCR inlet is established by the operational capability of the upstream diesel oxidation catalyst (DOC) and diesel particulate filter (DPF) since these components tend to make NO2 from NO upstream of the SCR catalyst inlet. However, as these components age, this ability to convert NO to NO2 deteriorates, and the molar ratio of NO2 to NOx at the inlet to the SCR catalyst drops. Thus, the efficiency of the SCR catalyst is reduced over time. Accordingly, further technological developments in this area are desirable.